A method of detecting a direction of a wave signal source, that is, a direction detection method generally uses an angle of arrival (AOA) method. A direction detection method using an amplitude or signal strength comparison, a virtual Doppler method, an interferometry method using a phase difference comparison, and the like, are used as the AOA method.
The amplitude or signal strength comparison method sets, as a direction, a point at which a signal strength is at a maximum in a state in which a directional antenna is being rotated or fixed. Thus, the larger an antenna size (the smaller 3 dB width), the better the accuracy.
The virtual Doppler method needs to generate a virtual Doppler by electrically rotating a circular array omni-directional antenna. Thus, the larger a rotation per minute (RPM) and the larger a rotation radius, the better the accuracy.
The basic concept of the interferometry method using the phase difference comparison is to find a direction of a signal as expressed by Equation 1 by measuring a phase difference between signals received by the interferometry based on two fixed omni-directional antennas as a single baseline as shown in FIG. 1. However, an array of a minimum of five antennas as shown in FIG. 2 is required to precisely find a direction, and a radio frequency (RF) apparatus or a portion for signal processing is complex. In Equation 1, denotes a wavelength, ΔΦ denotes a phase difference, φ denotes an AOA, and d denotes a distance between two antennas.
                              Δ          ⁢                                          ⁢          ϕ                =                              -                                          2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                d                            λ                                ⁢                      sin            ⁡                          (              φ              )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
That is, the amplitude or signal comparison method among AOA methods requires an antenna having an aperture surface having greater directivity to obtain a high precision. Thus, there is limitation on an antenna size. The virtual Doppler method requires thousands to tens of thousands electrical RPM, which leads to complexity and high cost. The fixed phase comparison method requires a minimum of five circular array antennas to detect the omni-directions (0 degrees to 360 degrees). The accuracy increases as the number of array antennas increases. However, as the number of array antennas increases, the number of RF elements increases and signal processing becomes complex.
As shown in FIG. 3, it is possible to increase the accuracy of direction detection with the phase comparison method using a log period dipole array (LPDA) that includes a single baseline including a log period (LP) antenna that is a directional antenna and two dipole antennas. In the related art, an approximate direction is found based on a signal strength by rotating the LP antenna, and a precise direction is found by measuring a phase difference between signals received by the dipole antennas around an angle found by the LP antenna. However, in the case of inflow of a severe reflected wave, a peak pattern of received signals by the LP antenna is distorted, thereby causing, an ambiguity error that an AOA value around zero degrees with respect to a zero-degree phase difference to be detected is not detected and an AOA value by other reflected signals is detected, as shown in FIG. 4.
Accordingly, proposed is a more precise and economical direction detection method.